Field of the Invention
The invention relates in general to receiving signals, and more particularly, to receiving orthogonal frequency division multiple access (OFDMA) signals.
Description of the Related Art
The number of standards and specifications that transmit signals in form of orthogonal frequency division multiple access (OFDMA) is increasing, especially in the transmission of wireless signals, such as Digital Video Broadcasting-Terrestrial (DVB-T), DVB-T2, and Integrated Services Digital Broadcasting (ISDB). In the above signal transmission standards, due to the lack of time interleaving transmission specifications, these signal transmission standards are easily influenced by impulsive interference.
FIG. 1 shows a schematic diagram of stereotypic impulsive interference. In FIG. 1, the time axis is not depicted in true ratios to emphasis a part to be explained by the present invention. As shown in FIG. 1, there are two bursts 110 and 120, spaced by a period of about 10 ms, along the time axis. The burst 110 includes four pulses 111 to 114, with an interval between the pulses being about 15 us to 35 us and each of the pulses lasting for about 250 ns. Further, the pulses have different strengths. Similarly, the burst 120 includes multiple pulses that are not shown.
FIG. 1 shows stereotypic impulsive interference. One person skilled in the art can understand that there are other types of impulsive interference. When encountering impulsive interference, frequency bands of all or at least a part of subcarriers of common OFDMA signals suffer from interference. As such, a receiver detects an increase in the signal strength for the interfered subcarrier frequency bands. The intensity of the impulsive interference is greater than the original additive white Gaussian noise (AWGN).
According to the DVB-T standard, the length of a symbol under a 2K mode is 224 us, and the length of a symbol under an 8K mode is 896 us. According to the DVB-T2 standard, the symbol length may be 112 us to 3584 us. In the stereotypic impulsive interference shown in FIG. 1, the time interval between two bursts is about 10 ms, which is far greater than the symbol length defined by the DVB-T and DVB-T2 standards. Further, the time interval between pulses is about 15 us and 35 us, which is noticeably shorter than the symbol length defined by the DVB-T and DVB-T2 standards. Further, the duration of each pulse is only about 250 ns. In other words, it is rare that two consecutive bursts suffer from the impulsive interference, and the burst may fall between a guard period between symbols or fall in a cyclic prefix.
FIG. 2A shows a schematic diagram of impulsive interference that falls in a cyclic prefix. In FIG. 2A, three OFDMA symbols 212, 222 and 232 respectively correspond to respective preceding cyclic prefixes 210, 220 and 230. A large part of a burst 220A falls in the cyclic prefix 220. Referring to FIG. 2B showing a schematic diagram of impulsive interference that falls in a cyclic prefix, another burst 220B falls in the symbol 212.
When encountering impulsive interference, the receiver needs to detect and recognize that the cyclic prefix of a particular symbol suffers from impulsive interference before being able to perform special processes on that symbol. Therefore, there is a need for a reliable mechanism for determining whether a symbol or a cyclic prefix of a symbol suffers from impulsive interference.